Bruton tyrosine kinase (BTK) is a member of the Tec family of protein tyrosine kinases. BTK has domains with pleckstrin homology (PH), Tec homology (TH), Src homology 3 (SH3), Src homology 2 (SH2), and tyrosine kinase or Src homology 1 (TK or SH1) (Akinleye et al., “Ibrutinib and novel BTK inhibitors in clinical development,” Journal of Hematology & Oncology, 2013, 6:59). In the normal development of B lymphocytes, the correct expression of BTK gene in different regions plays a key role in the function of B cells and various signal transduction pathways.
BTK functions downstream of multiple receptors, include B-cell Receptor (BCR), receptors for growth factors and chemokines, and innate immune receptors. BTK initiates a diverse range of cellular processes, such as cell proliferation, survival, differentiation, motility, adhesion, angiogenesis, cytokine production, and antigen presentation, and plays an important role in hematological malignancies and immune disorders (Kurosaki, “Molecular mechanisms in B cell antigen receptor signaling,” Curr OP Imm, 1997, 9(3):309-18). In a mouse model for chronic lymphocytic leukemia (CLL), BTK expression levels were shown to set the threshold for malignant transformation; BTK overexpression accelerated leukemia and increased mortality (Kil et al., “Bruton's tyrosine kinase mediated signaling enhances leukemogenesis in a mouse model for chronic lymphocytic leukemia,” Am J Blood Res 2013, 3(1):71-83).
Ibrutinib (also known commercially as IMBRUVICA®) was the first BTK inhibitor approved by the United States Food and Drug Administration for treating mantle cell lymphoma (MCL), chronic lymphocytic leukemia (CLL), and Waldenström's macroglobulinemia (WM). In general, however, the selectivity of known BTK inhibitors is not ideal—they inhibit not only BTK, but also various other kinases (such as ETK, EGF, BLK, FGR, HCK, YES, BRK and JAK3, etc.). Known BTK inhibitors also produce a variety of derivatives. These characteristics of known BTK inhibitors lead to a decrease in therapeutic efficacy and an increase in side effects. The pharmacokinetics of known BTK inhibitors also needs to be improved. Indeed, significant variations in bioavailability of ibrutinib have been observed clinically among patients (Marostica et al., “Population pharmacokinetic model of ibrutinib, a Bruton tyrosine kinase inhibitor, in patients with B cell malignancies,” Cancer Chemother Pharmacol, 2015, 75:111-121).